I definitely think there will be Sterek in season 3. It may or may not be anything romantic, but at least there will be Sterek interactions which I love because even the Sterek interactions from s1 and s2 felt a bit like they were working into becoming each other’s love interests.
I do think they’re keeping info about Sterek under wraps. Even if the info about Sterek isn’t anything super big. But I think they’re being careful because if they say anything positive about Sterek, then the Sterek fandom explodes with “OMG Sterek is going canon.” and if they say anything even slightly negative, then people explode with “OMG They’re queerbaiting us. They’re just tagging us along.” So it’s kind of a double sided blade and they probably want to keep the idea of Sterek happening at a 50/50 neutral.
As for the relationship between Stiles & Derek. Obviously it’s going to grow because Stiles is the detective this season and what do you think he’s going to be investigating? He’s probably going to be investigating why the alpha pack is in BH and why they’re trying to get Derek to join their pack. So Stiles/Derek kind of happens by default. And also, Jeff said that there would be a lot of Stiles/Peter scenes this season so that should bring him closer to the Hales.
I’m pretty sure Jeff was trolling when he said “what new girl?” So Derek is probably getting a girl. Dylan also confirmed that Stiles would be “toying” with a few girls, but idk if it will result in a female love interest for him.